Aging With Grace: The Nun Study and the Science of Old Age by David Snowdon 256pp, Fourth Estate, £16.99

How do you ask a nun for her brain? Medical researchers like to study religious groups, as their similar lifestyles give a purer picture of a disease. Still, there is something inescapably yucky about asking a 75-year-old nun to sign up for an Alzheimer's study in which she will be tested annually for signs of senility. When she does eventually depart this earthly realm, her soul will be headed one way but her brain will be raced across country, bundled into a plastic tub and freighted parcel express to a laboratory for analysis.

Could there be any more morbid subject matter for a book? Yet David Snowdon, a University of Kentucky epidemiologist, has written a wonderfully warm and illuminating account of his now famous Nun Study. Aging With Grace is lucid about the science of senility - what exactly are the chances of escaping dementia? And it is reassuringly honest about the business of doing such science - what happens when a nervous academic has to explain the nature of his work to a roomful of twinkly-eyed ladies all old enough to be his granny?

When he first met his nuns, Snowdon admits he did not even know what he was going to study. He was merely a young researcher desperate for a project that would get funded. A student introduced him to the local convent of the School Sisters of Notre Dame, and he realised that with their identical lifestyles and meticulous record-keeping they made an ideal group. Nuns had already contributed to important research into breast and cervical cancer, but he was not sure what fields that left him.

An answer soon became clear. In the 1980s, big bucks began to pour into Alzheimer's research. The western world's population was greying; the US Census Bureau predicted that there would be nearly 1m Americans over the age of 100 by 2050 - a 20-fold increase. This is an alarming prospect if great age turns the majority of us into drooling, incontinent, bed-ridden simpletons.

From his first visits to the convent, Snowdon saw the starkly contrasting fates that life can deal. Many of the nuns were in fine fettle, even into their 80s and 90s. He found them cheering the baseball on TV, working out in the convent gym, penning letters to their Congress representatives. The majority had worked as teachers at Catholic schools and remained mentally active in retirement. But a walk to the hospital wing told a different story. There nuns were slumped blankly in wheelchairs or gabbling in unintelligible "word soup".

As the nuns had similar education, health care, diets and much else, Snowdon eventually realised he could say something about what might determine an individual's neurological destiny. The only problem: he would have to ask the nuns for their brains. With great candour, Snowdon describes the bumbling way he broke the idea to the sisters, and their surprisingly selfless response. So far he has signed up about 700 nuns to the study, which has become a nationwide enterprise. He has already made some startling findings.

Alzheimer's affects over a third of people who reach 85. It is a degenerative disease marked by plaques and tangles - protein clots and twisted filaments - that gradually kill brain cells. In people with a genetic predisposition, these plaques and tangles can start to spot the brain even in their 20s. And yet Snowdon found that Alzheimer's is certainly not inevitable. One nun lived to 100 with no trace of damage. Even more astonishingly for conventional views of the disease, he found that about a third of the nuns had advanced Alzheimer's - their brains were riddled with end-stage damage - yet had tested as mentally normal until the day they died. Somehow, their minds had remained intact.

One factor was that these "escapees" had avoided other forms of brain damage. Snowdon discovered that concussion or even very small strokes can hugely amplify the effects of any existing Alzheimer's deterioration. One finding that was harder to explain was that the writing style of the nuns in their youth predicted their mental health in their dotage. When entering the sisterhood in their 20s, each nun had to pen an autobiographical account of her reasons. It proved that those who wrote in simple, list-like sentences fared the worst. Those who used lush, idea-dense prose turned out to be the chirpy octogenarians. This effect seemed independent of simple intelligence: perhaps a brain that begins life making richer patterns of connection can better survive the pock-marking effects of Alzheimer's? Snowdon has to admit that he doesn't yet know.

This is a rare book for the way it combines cutting-edge science with an inside view of how that knowledge is being won. It is as much a story of the individual lives of the nuns as of their eventual neurological fate. And the results are not yucky at all, but encouragingly heart-warming.

 John McCrone's How The Brain Works will be published by Dorling Kindersley in 2002.